In microelectromechanical systems (MEMS), the functional element, such as a circuit, sensor or actuator, must often be separated from the environment, yet electrical signals must be sent to and received from the functional element. The functional elements are flexible structures operable within a hermetically sealed cavity. The challenge in packaging MEMS is achieving the electrical connection without opening the cavity to the environment, and to do this with a simple process that results in a small overall footprint and outline. In other words, the MEMS should be fabricated to form a functional element within a hermetically sealed cavity in the semiconductor wafer having an overall structure of small dimension both laterally and vertically.
Past and current attempts to achieve such MEMS include die-level hermetic packaging, but this method produces a device with a large package size at a high cost. Another example is a buried feed-through, but this is a relatively complex process. Still another example is first bonding, then sealing open areas with high temperature LPCVD films, but high temperature processing is not compatible with many metals present on the wafers.
A need thus exists in the semiconductor industry for a method for wafer-level packaging for MEMS applications that allow small footprints, small chip outlines, low-temperature processing, hermetic sealing, and low cost.